The present invention relates generally to exhaust gas-driven turbochargers, and relates more particularly to exhaust gas-driven turbochargers having a variable turbine nozzle of the axially sliding piston type for varying the size of the nozzle that leads into the turbine wheel so as to regulate flow into the turbine wheel.
Regulation of the exhaust gas flow through the turbine of an exhaust gas-driven turbocharger provides known operational advantages in terms of improved ability to control the amount of boost delivered by the turbocharger to the associated internal combustion engine. The regulation of exhaust gas flow is accomplished by incorporating variable geometry into the nozzle that leads into the turbine wheel. By varying the size of the nozzle flow area, the flow into the turbine wheel can be regulated, thereby regulating the overall boost provided by the turbocharger's compressor.
Variable-geometry nozzles for turbochargers generally fall into two main categories: variable-vane nozzles, and sliding-piston nozzles. Vanes are often included in the turbine nozzle for directing the exhaust gas into the turbine in an advantageous direction. Typically a row of circumferentially spaced vanes extend axially across the nozzle. Exhaust gas from a chamber surrounding the turbine wheel flows generally radially inwardly through passages between the vanes, and the vanes turn the flow to direct the flow in a desired direction into the turbine wheel. In a variable-vane nozzle, the vanes are rotatable about their axes to vary the angle at which the vanes are set, thereby varying the flow area of the passages between the vanes.
In the sliding-piston type of nozzle, the nozzle may also include vanes, but the vanes are fixed in position. Variation of the nozzle flow area is accomplished by an axially sliding piston that slides in a bore in the turbine housing. The piston is tubular and is located just radially inwardly of the nozzle. Axial movement of the piston is effective to vary the axial extent of the nozzle leading into the turbine wheel, thus varying the “throat area” at the turbine wheel inlet. When vanes are included in the nozzle, the piston can slide adjacent to radially inner (i.e., trailing) edges of the vanes; alternatively, the piston and vanes can overlap in the radial direction and the piston can include slots for receiving at least a portion of the vanes as the piston is slid axially to adjust the nozzle.
There are times during the operation of a turbocharger when it is desired to pass as much flow through the turbine housing as possible. For example, it may be desirable to minimize the backpressure felt by the engine during certain operating conditions, and this is accomplished by reducing the flow restriction downstream of the engine as much as possible. In a sliding piston-type variable turbine nozzle, the downstream flow restriction is reduced by fully opening the piston to maximize the throat area at the turbine wheel inlet. However, in some cases, even fully opening the piston may not allow as much flow to pass as may be desired.